As a conventional bone conduction device, one shown in FIG. 8 has been known in the art. A bone conduction speaker of such a conventional bone conduction device has a construction in which: provided is a yoke 51 provided with a center magnetic pole 52, on which pole 52 a voice coil 56 is wound and mounted thereon; the yoke extends in four directions to form four extension parts, disposed on opposite two ones 53, 53 of which parts are magnets 55, 55, wherein the remaining two ones of the extension parts carry tympanum fixing parts 54, 54 disposed upright on these parts; and, the tympanum 57 is fixedly mounted on each of the tympanum fixing parts 54, 54 (Japanese Patent No. 2967777).
On the other hand, a bone conduction speaker shown in FIG. 9 comprises: a cylindrical case 61 provided with a tympanum 62 in its upper surface; a cylindrical magnet 63 fitted in the cylindrical case 61 to extend along an inner surface of the case 61; a yoke 65, which is provided with a center magnetic pole 64 in its center and fitted in a lower part of the case 61; and, a voice coil 66 disposed so as to circumscribe the center magnetic pole 64 of the yoke 65 (Japanese Patent application Laid-Open No.: Sho 57-10114 gazette).
Another bone conduction speaker shown in FIG. 10 is known, wherein: provided in opposite sides of a yoke 70 are a pair of branched legs 71, 71 each of which is fitted in each of voice coils 72, 72; a magnet 73 is disposed between the voice coils 72, 72; and, a tympanum 75 is disposed on the magnet 73 through a plate 74 (Japanese Patent application Laid-Open No.: 2001-313989 gazette). This is one in which two pieces of the voice coils 72 are disposed, so that this one generally larger in size than the above-described one.
In recent years, the bone conduction speaker described above has been used in a condition in which the bone conduction speaker is incorporated in a cellular phone. Under such circumstances, since a need exists in the latest cellular phone for increasing in area size a display screen in a front surface of the cellular phone as large as possible, a space available for installation of a speaker in the cellular phone is limited only to a laterally elongated space positioned over the display screen.
However, in case of the conventional type of bone conduction speaker described above, any one of the speakers has an aspect ratio of approximately 1:1 to assume a circle-like form. Due to this, it is necessary for the speaker to be a scaled-down one, a diameter of which is smaller in length than a shorter side of a laterally elongated installation space appearing in an upper part of a case of a cellular phone. This leads to a shortage in output power of the bone conduction speaker. In order to use a large-sized bone conduction speaker which is capable of providing a sufficient amount of output power in use, it is necessary to scale-up the case of the bone conduction speaker in size. In this case, however, there is a fear that a considerable loss in space occurs in the cellular phone.
The above description is also true in the case of a bone conduction microphone, which must be incorporated in a limited installation space.    Patent document 1: Japanese Patent No.: 2967777;    Patent document 2: Japanese Patent application Laid-Open No.: Sho 57-10114 gazette; and    Patent document 3: Japanese Patent application Laid-Open No.: 2001-313989 gazette.